1. Field of the Invention
This invention relates to a traction control system for a vehicle, and more particularly to a traction control system for a vehicle for preventing side slip of the vehicle during a turning.
2. Description of the Prior Art
There has been known a traction control system for a vehicle which controls the traction of the vehicle in order to prevent deterioration of the accelerating performance due to slip of the driving wheels caused by an excessive driving torque, for instance, during acceleration.
In the traction control, generally, the rate of slip of the driving wheels is determined as the difference between the driving wheel speed (the rotational speed of the driving wheels) and the vehicle speed calculated on the basis of the rotational speed of the driven wheels, and the engine output is controlled by fuel injection control and/or ignition timing control so that the rate of slip of the driving wheels converges on a predetermined target value.
However since the rate of slip of the driving wheels is determined solely depending on the speed of rotation of the driving wheels about the drive shaft, side slip of the vehicle due to the lateral acceleration generated during a turning cannot be satisfactorily prevented by the conventional traction control system.
That is, when the vehicle is making a turn, a lateral acceleration directed outward of the turn is generated on the vehicle body and the vehicle is apt to slip sideways. Accordingly, when the engine output is controlled during a turning with a control variable whose value is the same as during a straight running, the output torque of the engine is kept equal to that during a straight running and the gripping force of the wheels is not changed though a larger gripping force of the driving wheels is required to resist the lateral acceleration.
Further when the vehicle is running straight, the driving force of the vehicle acts as a forward propelling force as it is. However when the vehicle is making a turn, the driving force is divided into the outward lateral acceleration and the forward propelling force, which result in reduction of the forward propelling force and reduction in the rotational speed of the driving wheels. When the rotational speed of the driving wheels lowers, the rate of slip of the driving wheels as calculated on the basis of the rotational speed of the driving wheels reduces and the control variable of the engine output becomes smaller as compared with when the vehicle is running straight, whereby the engine torque is less reduced during a turning. This problem takes place in parallel to the aforesaid problem and accordingly, it becomes difficult to stably steer the vehicle along a curve.